Temporal fluctuations in the microcirculation have long been considered the result of active biological control. Recent computer simulations indicate that the microvascular networks can exhibit nonlinear dynamics. Spontaneous sustained and damped oscillations can occur as well as steady states. The simulations are based on well established blood rheological properties; the Fahraeus-Lindqvist effect and plasma skimming. Realistic network geometries based on in vivo observations of rat mesentery are used in the simulations. The proposed research will provide experimental verification of these model predictions. In vitro replicas of small microvascular networks will be perfused with red blood cell suspensions under conditions predicted to permit oscillations. Oscillations will be monitored by pressure measurements. Both tree- and arcade type networks will be studied. In vitro methods will be used to avoid biological control activity during the experiments. Numerical simulations are also planned to determine the influence of parameters such as vessel diameters, lengths, hematocrits, red cell residence times, etc on the amplitude and frequency of the oscillations. Numerical research will also explore the possibility of period doubling and chaotic fluctuations as parameters in the problem are varied. The anticipated results will provide a nonlinear dynamic model through which the concepts of biological control mechanisms need to be viewed. Oscillating flows and pressures due to nonlinear effects will also have an impact on mass transfer calculations from blood to surrounding tissues. Time varying flows are also implicated in the health of the vessel wall.